The present invention relates to a grinder for grinding process, and more particularly to structure for a grinding head for a surface grinder for super-precision grinding process.
There are three types of surface grinders. In a first type, a column secured to a bed is provided for supporting a grinding head having a grinding-wheel spindle and guiding along a vertical path. A table to which a workpiece is secured is moved in a cross direction and a longitudinal direction. In a second type, a column is provided on a cross slide which is moved in a cross direction. The column has a grinding head which is moved in a vertical direction. A table is moved in a longitudinal direction only. A third type is a portal type surface grinder, in which a cross rail is mounted on a portal column and provided to be moved in the vertical direction. A grinding head is provided on the cross rail to be moved in a cross direction. A table is moved in a longitudinal direction.
In the first and second types, since the workpiece on the table is fed passing through a space under the grinding head in the cross direction, or the grinding head moves in the cross direction, a grinding wheel must be located at a position largely apart from the column by a width of the table in cross direction. Therefore, a grinding-wheel spindle has a cantilevered structure projected from a bearing portion. In the third type, the grinding wheel is mounted on an end of the grinding wheel spindle. Thus, the spindle also has the cantilevered structure. The cantilevered structure is inherently inferior in stiffness. In order to improve the accuracy of grinding process, even if stiffness of bearing elements for the grinding wheel spindle is increased, the accuracy cannot be sufficiently improved, because of deflection of the cantilevered spindle. To avoid such a deflection, stiffness of the spindle must be increased. The stiffness of the spindle can be increased by enlarging the diameter. However, the diameter of the spindle cannot be largely increased, since the large diameter portion interferes with the work. Therefore, the increase of the stiffness of the spindle is limited.
In addition, thermal displacement of elements of the grinding machine is increased at the grinding process position, because of the cantilevered structure. Accordingly, high accurate grinding process cannot be expected in conventional grinders.